The present invention relates to a wafer assembly having a switch mechanism that, in one embodiment, selectively and alternately electrically connects a suspended element or connection with the probe assembly.
Testing of electrical devices is accomplished by routing a test signal through a probe assembly to selected contact points on a device under test (DUT). Measurement devices are then used to determine the voltages and/or currents generated in the DUT in response to that test signal.
When probing a DUT, it is highly desirable to protect the probe assembly from electrical interference that can result, for example, from dynamic electromagnetic fields present in the immediate vicinity of the probe assembly. To reduce this type of interference, probe assemblies commonly provide the test signal through a coaxial or triaxial cable so that the test signal may be guarded and/or shielded. In this arrangement, the center conductor of the coaxial or triaxial cable carries the test signal. A guard signal, preferably substantially identical to the test signal, or otherwise “follows” the test signal, is routed through either the outer conductor of a coaxial cable, or the middle conductor of a triaxial cable. If a triaxial cable is employed, a shield signal, which is typically instrumental “ground” or another relatively constant potential, may be provided through the outer conductor.
The DUT typically rests upon the top layer of a multilayer electrically conductive chuck where each layer is insulated from the others. The guard may be connected to the second layer of the chuck and, if desired, a conductive plate may laterally surround the chuck. The shield may be connected to the third layer of the chuck.
It is desirable to closely match the signals between the test signal and the guard. Because the shield is exposed to external interference along its outer periphery it will not ordinarily match precisely the guard signal it encloses. The resulting discrepancy between the shield and guard signals permits some interference to affect the guard signal. The modified guard signal in turn affects the test signal. Usually, the resulting interference is insignificant. However, when the test signal to the DUT is very small, even this amount of interference can be significant.
When small amounts of interference are significant, further reduction in the interference can be obtained by locating a suspended conductive plate over the DUT and connecting it to the guard signal. The suspended conductive plate defines a central opening so that the probe assembly may make electrical contact with the DUT. In this fashion, the DUT may be guarded from all sides by signals closely approximating that delivered to the DUT.
Another problem that occurs when testing a DUT with a probing assembly is that chuck and/or cable impedances can often inhibit accurate measurement of the voltages and/or currents present in response to the applied test signal. This problem is particularly prevalent with high-current test signals where the voltage drop along the cable and/or through the chuck can be significant.
The typical adaptation to this problem is to connect a second coaxial or triaxial cable to the probe assembly in the same manner as the first. In this two-cable configuration, commonly referred to as a Kelvin connection, one cable provides the “force” (e.g. a low impedance connection that primarily supplies a current) while the other cable provides a “sense” (e.g. a high impedance connection that primarily senses a voltage) used to measure the resulting voltages and/or currents present within the DUT. In a “true” Kelvin connection, both the force and the sense comprise independent paths from the test equipment to the DUT. Alternatively, both the force and the sense lines may be connected at a common interconnection point, that is as close to the DUT as possible, frequently referred to as a quasi-Kelvin connection.
Designing a probe assembly that provides both a suspended guard and a Kelvin connection has recently become problematical because of the total capacitance seen by the measurement equipment. Measurement equipment that is used in conjunction with the probe assembly only provides valid measurements so long as total circuit capacitance remains within specified parameters. In addition, testing equipment imposes capacitance restrictions between the force and guard, and the guard and shield, respectively.
Historically, probe assemblies containing both a suspended guard and a Kelvin connection could be designed within the capacitance limitations set by the testing equipment. In recent years, the industry has moved towards manufacturing individual electrical devices, such as silicon semiconductor components, on a larger-sized 12″ wafer. Any probe assembly that tests a DUT on such a wafer must therefore have a larger chuck, which adds capacitance to the probe assembly. The available capacitance permitted to be connected to the instrumentation that is not used by the chuck now must be rationed carefully. Both a suspended guard and a Kelvin connection add capacitance to the test circuit loop, and it has proven difficult to design a probe assembly that contains both elements, yet remains within the capacitance restrictions set by the measurement equipment.
What is desired, therefore, is a probe assembly that may accurately test electrical components and that includes a suspended guard and another connection.